Barrier layers are used to inhibit diffusion. They reduce permeation through a coated substrate. Frequent applications are found wherever the object is to prevent certain substances, e.g., foodstuffs as goods for packaging, from being able to come into contact with oxygen from the environment or water from being able to exchange with the environment. The primary focus is thereby on an oxidative reaction or perishability of the substances to be protected. In addition, i.a., the protection of various oxidation-susceptible substances when they are integrated into layer composites is also taken into consideration. The protection of these substances is of particular importance when retarding the oxidative reaction determines the life of products.
Barrier layers pose in part a very differing resistance to different diffusing substances. The permeation of oxygen (OTR) and water vapor (WVTR) under defined conditions through the substrate provided with the barrier layer is often cited to characterize barrier layers. Moreover, barrier layers often have the function of an electric insulating layer. Display applications represent an important field of application for barrier layers.
Through coating with a barrier layer, permeation through a coated substrate is reduced by a factor that can lie in the single-digit range or can be many orders of magnitude. Within the meaning of the invention, ultrabarrier layers are understood to be layers, the barrier effect of which prevents permeation values of WVTR=0.05 g/m2d and OTR=0.2 cm3/m2d from being exceeded (WVTR according to DIN 53122-2-A; OTR according to DIN 53380-3).
In addition to preset barrier values, various other target parameters are often expected of a finished barrier layer. Examples thereof are optical, mechanical and technological-economic requirements. Barrier layers are often required to be invisible, therefore must be virtually completely transparent in the visible spectral range. If barrier layers are used in layer systems, it is often advantageous if coating steps for applying individual parts of the layer system can be combined with one another.
Cathode sputtering methods, so-called sputtering methods, have an important place among the coating methods that are used in the production of layer systems, since sputtering methods make it possible to deposit layers of high quality. It is therefore often desirable in the production of layer systems to be able to use sputtering methods at least in combination with other coating methods.
So-called PECVD methods (plasma-enhanced chemical vapor deposition) are often used to produce barrier layers. These are used for various substrates for different layer materials. It is known, for example, to deposit SiO2 and Si3N4 layers of a thickness of 20 to 30 nm on 13 μm PET substrates [A. S. da Silva Sobrinho et al., J. Vac. Sci. Technol. A 16(6), November/December 1998, p. 3190-3198]. Permeation values of WVTR=0.3 g/m2d and OTR=0.5 cm3/m2d can be achieved in this manner at an operating pressure of 10 Pa.
An oxygen barrier of OTR=0.7 cm3/m2d can be realized with coating using SiOx for transparent barrier layers on PET substrate by means of PECVD [R. J. Nelson and H. Chatham, Society of Vacuum Coaters, 34th Annual Technical Conference Proceedings (1991), p. 113-117]. For transparent barrier layers on PET substrate, other sources on this technology also assume permeation values in the order of magnitude of WVTR=0.3 g/m2d and OTR=0.5 cm3/m2d [M. Izu, B. Dotter, S. R. Ovshinsky, Society of Vacuum Coaters, 36th Annual Technical Conference Proceedings (1993), p. 333-340].
Disadvantages of the known PECVD methods lie above all in obtaining relatively low barrier effects. This makes the products unattractive for display applications in particular. Another disadvantage is the high operating pressure that is necessary to implement this method. If a coating step of this type is to be integrated into complex production processes in vacuum units, a high expenditure for pressure decoupling measures may be necessary. A combination with sputtering processes in particular is usually uneconomical for this reason.
It is known to apply barrier layers by sputtering. Sputtered individual layers often show better barrier properties than PECVD layers. For example, WVTR=0.2 g/m2d and OTR=1 cm3/m2d are given as permeation values for sputtered AlNO on PET [Thin Solid Films 388 (2001) 78-86]. In addition, numerous other materials are known that are used to produce transparent barrier layers in particular through reactive sputtering. However, the layers thus produced likewise have barrier effects that are too low for display purposes. Another disadvantage of such layers is their low mechanical load capacity. Damage occurring through technologically unavoidable stresses during further processing or use mostly lead to a marked deterioration of the barrier effect. This makes sputtered individual layers often unusable for barrier applications.
It is known to vapor-deposit individual layers as barrier layers. Through this PVD method, different materials can likewise be deposited directly or reactively on various substrates. For example, the reactive vapor deposition of PET substrates with Al2O3 is known for barrier applications [Surface and Coatings Technology 125 (2000) 354-360].
Permeation values of WVTR=1 g/m2d and OTR=5 cm3/m2d are hereby achieved. These values are likewise much too high to use such coated materials as barrier layers in displays. They are often even less loadable in mechanical terms than sputtered individual layers. Moreover, a direct vaporization is usually associated with a high vaporization speed or rate. With the production of thin layers customary in barrier applications, this requires correspondingly high substrate speeds in order to avoid the substrate being loaded too much A combination with process steps that require a much lower throughput speed is thus virtually impossible in continuous pass plants. This affects in particular the combination with sputtering processes.
It is known that the mechanical stability of inorganic vapor-deposited layers can be improved if an organic modification is carried out during vaporization. The integration of organic constituents into the inorganic matrix forming during the layer growth thereby occurs. Apparently, an increase in the elasticity of the entire layer occurs through the incorporation of these further constituents into the inorganic matrix, which markedly reduces the danger of fractures in the layer. A combination process that combines an electron-beam evaporation of SiOx with the admission of HMDSO is given in this context as an example of one suitable at least for barrier applications (DE 195 48 160 C1). However, low permeation rates necessary for display applications cannot be obtained with layers produced in this manner. Another drawback is that the electron-beam evaporation requires the high coating rates mentioned, which makes a combination with many other process steps much more difficult.
It is known to apply barrier layers in several coating steps. One method is the so-called PML process (polymer multilayer) (1999 Materials Research Society, p. 247-254); [J. D. Affinito, M. E. Gross, C. A. Coronado, G. L. Graff, E. N. Greenwell and P. M. Martin, Society of Vacuum Coaters, 39th Annual Technical Conference Proceedings (1996), p. 392-397]. In the PML process a liquid acrylate film is applied to the substrate by means of vaporizers, which film is hardened by means of electron-beam technology or UV irradiation. This does not have a particularly high barrier effect per se. Subsequently, a coating of the hardened acrylate film takes place with an oxide intermediate layer onto which in turn an acrylate film is applied. This procedure is repeated several times as needed. The permeation values of a layer stack thus produced, i.e., a combination of individual acrylate layers with oxide intermediate layers, lies below the limit of measurement of conventional permeation measuring instruments.
There are disadvantages above all in the necessary use of complex systems engineering. It is imperative for vacuum units to operate according to the multi-chamber principle, which is associated with a high price. Moreover, first a liquid film forms on the substrate, which film has to be hardened. This leads to an increased soiling of the equipment, which shortens maintenance cycles. The process is likewise optimized for high line speeds and is therefore difficult to combine with slower coating processes, in particular with a sputtering process, in an in-line manner.
It is further known to use magnetron plasmas for a plasma polymerization in the deposit of diffusion barrier layers, i.e., barrier layers (EP 0 815 283 B1); [S. Fujimaki, H. Kashiwase, Y. Kokaku, Vacuum 59 (2000), p. 657-664]. These are PECVD processes that are directly maintained through the plasma of a magnetron discharge. An example of this is the use of a magnetron plasma for PECVD coating to deposit layers with a carbon skeleton, whereby CH4 is used as a precursor. However, layers of this type likewise have insufficient barrier effect for display applications.